Introduction Joint trauma leads to acute posttraumatic arthritis and in the majority of individuals, as a long-term complication, to osteoarthritis (OA) [1].  ere are an estimated 900,000 cases of knee injuries annually in the United States, and posttraumatic OA accounts for 12% of all cases of OA [2]. In some joints, such as the ankle, OA predominantly develops after joint trauma [2]. As posttraumatic OA primarily aff ects younger individuals [3,4], it leads to reduced physical activity and to deconditioning of the musculoskeletal system. Joint replacement in this young patient group is complicated by the limited lifespan of the implants. OA risk increases with patient age at the time of injury and with time from the onset of injury [4,5].  e presence of additional OA risk factors, such as obesity, joint malalignment or genetic risk factors, leads to a more severe outcome. Between 60 and 80% of patients with magnetic resonance imaging or arthroscopically docu- mented cartilage injury developed cartilage degeneration within 5 years [6,7]. Patients with anterior cruciate liga- ment (ACL)-defi cient knees, with or without a concomi- tant meniscus injury, are at high risk for posttraumatic OA [5,8]. Previous concepts that residual joint instability after ACL reconstruction is the cause of OA have not been confi rmed as OA develops in joints with ACL injuries even if reconstructive surgery successfully nor mal izes joint biomechanics.  ese observations empha size the role of events in the time period after the initial joint trauma. Joint trauma aff ects all joint tissues to some degree but the damage to articular cartilage appears most signifi cant, as it is largely irreversible and may be the major deter- minant for the subsequent development of OA.  ere is a certain degree of immediate or irreversible damage, but the days and weeks after injury represent the phase where damage progresses most rapidly.  e acute symptoms following joint injury include joint pain and swelling due to intraarticular bleeding, synovial eff usion and infl am- ma tory cell infi ltration. Patients typically undergo surgical treatment of the ligament and meniscus lesions within 3months after the initial injury [2]. Currently there are no approved therapies to address acute posttraumatic arthritis. Corticosteroids have potent anti-infl ammatory activity but potential benefi ts or adverse eff ects of cortico steroids in a restricted dose and frequency of administration for traumatic joint injury have not been resolved and remain to be studied. Furthermore, measures to prevent OA are not available, although patients with posttraumatic arthritis represent a readily identifi ed population at risk for developing OA and thus are ideal to test preventive and therapeutic measures. Interventions early during the most dynamic postinjury phase have the potential to limit the degree of acute joint damage and to delay the onset and reduce the severity of OA.  e prolonged posttraumatic infl amma- tory insult also signifi cantly increases the risk of arthro- fi brosis for which satisfactory management remains to be developed.  e present review addresses pathogenetic Abstract Joint trauma can lead to a spectrum of acute lesions, including osteochondral fractures, ligament or meniscus tears and damage to the articular cartilage. This is often associated with intraarticular bleeding and causes posttraumatic joint in ammation. Although the acute symptoms resolve and some of the lesions can be surgically repaired, joint injury triggers a chronic remodeling process in cartilage and other joint tissues that ultimately manifests as osteoarthritis in a majority of cases. The objective of the present review is to summarize information on pathogenetic mechanisms involved in the acute and chronic consequences of joint trauma and discuss potential pharmacological interventions. The focus of the review is on the early events that follow joint trauma since therapies for posttraumatic joint in ammation are not available and this represents a unique window of opportunity to limit chronic consequences. © 2010 BioMed Central Ltd Posttraumatic osteoarthritis: pathogenesis and pharmacological treatment options Martin K Lotz* REVIEW *Correspondence: mlotz@scripps.edu Department of Molecular and Experimental Medicine, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 © 2010 BioMed Central Ltd mechanisms and mediators involved in the acute and chronic consequences of joint trauma and candidates for pharmacological intervention. Pathogenetic mechanisms  e pathogenetic processes can temporally be separated into the immediate events that are related to the mechanical impact, the acute posttraumatic phase with prominent infl ammation that can last up to approxi- mately 2 months and the chronic phase. Subtle metabolic changes in cartilage and other joint structures slowly progress through a long clinically asymptomatic latency period to a symptomatic phase with joint pain and dysfunction. In the majority of patients this leads to a clinical diagnosis of OA, and in some patients ultimately requires joint replacement (Table 1). Immediate e ects of mechanical impact  e acute mechanical overload during joint trauma can cause bone fracture, rupture of ligaments and menisci, lesions in the joint capsule and synovium, and compres- sive or shear damage to the articular cartilage. When cartilage is exposed to compressive and shear forces it can separate from the subchondral bone. Exposure to lower forces leads to immediate changes in cartilage cell viability due to necrosis, and cracks or fi ssures of the cartilage surface that can extend into the mid and deep zone, and leads to release of cartilage extracellular matrix molecules [9]. Compressive chondral injuries may not be evident at arthroscopy but are in some cases associated with subchondral bone marrow edema [10].  e synovial fl uid is severely compromised in its lubricating function.  is is the result of dilution due to intraarticular bleeding and plasma extravasation, leading to lower concentrations of hyaluronic acid and lubricin, the major joint lubricants. Neutrophil-derived enzymes degrade lubricin, and infl ammatory mediators present in the posttraumatic synovial fl uid suppress the synthesis of lubricin [11]. In patients with ACL injury, the decrease in lubricin is most marked in the days after injury and gradually approaches near-normal levels within 1 year [11].  e immediate collagen damage in cartilage is caused by mechanical rupture due to tensile failure [12]. Carti lage swelling occurs within hours after impact as the swelling pressure of the glycosaminoglycans (GAGs) is no longer restrained by an intact collagen network [13].  ere is also rapid GAG loss that appears to result from the acute physical impact since it is not prevented by inhibitors of GAG-degrading enzymes [14]. Following these immediate changes is the acute post- traumatic phase, with activation of remaining viable cells in articular cartilage and other joint tissues that respond to the mechanical trauma with enhanced cell metabolism and the generation of oxygen radicals, matrix-degrading enzymes and infl ammatory mediators. Mechanical injury also leads to suppression of collagen and GAG synthesis. A recovery from this suppression and an increase of new matrix synthesis can occur subsequently, but this is compromised by the presence of the infl ammatory response [15]. Hemarthrosis Rapidly developing intraarticular bleeding caused by rupture of blood vessels in the joint capsule, synovium, menisci or subchondral bone is observed in >90% of patients with joint trauma that have surgically signifi cant lesions such as osteochondral fracture, ligament or meniscus tears, but also occurs at lower frequency in patients without signifi cant acute intraarticular pathology [16]. Hemarthrosis is an important factor in the patho- gene sis of posttraumatic arthritis since even a single epi- sode of intraarticular bleeding can lead to cartilage damage. Experimental injection of autologous blood into normal joints causes loss of proteoglycans and inhibits proteoglycan synthesis [17]. In addition, exposure of articular cartilage in vitro to whole blood in the absence of other stimuli induces chondrocyte apoptosis. Neutro- phils in acute hemarthrosis are activated and produce increased levels of reactive oxygen species, elastase and other lysosomal enzymes [18]. Extracellularly released elastase is a potent lysosomal enzyme that degrades proteoglycans. Mononuclear cells cause reversible sup- pres sion of GAG synthesis but this becomes irreversible in the presence of red blood cells.  is irreversible inhibition is independent of the cytokines IL-1 and TNF Table 1. Pathogenesis of posttraumatic cartilage degradation Immediate (seconds) Acute (months) Chronic (years) Cell necrosis Apoptosis Joint tissue remodeling Collagen rupture Leukocyte in ltration Glycosaminoglycan loss In ammatory mediators Hemarthrosis Extracellular matrix degradation In ammation De cient lubricants Arthro brosis Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 2 of 9 but is in part dependent on oxygen radicals [19]. Hemo- globin degradation products such as deoxyhemoglobin, methemoglobin, and hemosiderin appear to mediate the blood-induced damage. Hemarthrosis also leads to synovial hypertrophy and siderosis, due to phagocytosis of erythrocytes and hemoglobin by synovial cells. Synovitis develops only at later stages and may be triggered by mediators that result from cartilage damage, such as matrix degradation products or chondrocyte-derived cytokines [20].  ere is thus strong evidence that intraarticular bleeding, even a single episode, leads to joint damage – and intraarticular bleeding should therefore be addressed in the treatment of posttraumatic arthritis. Additional bleeding at the time of surgery could in itself be deleterious to cartilage health and could potentially recapitulate and prolong the events initiated by the primary trauma. Arthro brosis Fibrogenesis resulting in clinically signifi cant arthro- fi brosis remains a problem due to the lack of effi cient preventive and therapeutic strategies [21,22]. Presently, clinical management of arthrofi brosis emphasizes pre ven- tion strategies, including early passive range-of-motion exercises. Once fi brosis has developed, interventions consist of steroid injections, physical therapy and, ultimately, surgery for debridement. Arthrolysis surgery may be required more than once in some patients. A key strategy for the prevention of arthrofi brosis is to delay the time to ACL reconstruction surgery for an acute ACL tear.  is approach is supported by evidence that performing surgery within 4 weeks of ACL injury is a risk factor for postoperative development of arthro- fi brosis [23].  e presence of preoperative swelling, eff usion and hyperthermia correlated with development of arthrofi brosis [24]. Furthermore, if joint infl ammation persisted after 4 weeks, the risk of arthrofi brosis re- mained elevated.  ese observations suggest it is infl am- mation, and not timing of the surgery, which predicts development of arthrofi brosis postoperatively. Attempts to reduce preoperative infl ammation are therefore requir ed to prevent this postsurgical complication. Posttraumatic cartilage cell death Cell death in cartilage has been identifi ed as an important mechanism in the development of OA joint pathology [25]. Cell death has also become a focus of research on posttraumatic cartilage damage and has been studied in vitro, in open and closed impact animal models, as well as in human joints. Cell death after traumatic cartilage impact occurs in two phases: an immediate phase due to cell necrosis, followed by a subsequent spreading of cell death media- ted by apoptotic mechanisms beyond the initial area to the surrounding unimpacted regions [12,26-29], leading to expansion of the original lesion [9].  is progressive increase in apoptotic cells after injury off ers a therapeutic window. Compressive loading of cartilage causes signifi - cant apoptotic cell death [26,30] that develops around matrix cracks, and there is a linear relationship between impact energy and cell death [31].  e cartilage super- fi cial zone is most susceptible to cell death after mecha- nical injury [32]. Apoptosis has been demonstrated after mechanical injury in animal models and in human joint trauma as indicated by the activation of caspases, the enzymes that regulate and execute apoptosis [10]. A sub- stantial increase in apoptotic cell death in cartilage was also observed after intraarticular fracture in humans [33,34].  e consequences of cell death are that it contributes to matrix degradation and depletes cartilage of the cells that are required to repair and maintain extracellular matrix.  e percentage of apoptotic chondrocytes corre- lates with the level of GAG loss in the impacted tissues [35].  is suggests that cell death contributes to matrix degradation and defi cient repair. Observations on the short-term consequences of mechanical cartilage injury on apoptosis thus suggest that: in vivo chondrocyte apoptosis can be induced by a single impact load; the extent of apoptosis in vitro correlates with the intensity of the load applied and increases with time in culture; chondrocyte death can precede structural damage; caspase inhibitors reduce cell death, maintain functional cells and protect against extracellular matrix damage; and a therapeutic window exists where apoptosis can be inhibited. In ammatory cytokines Cytokines in the IL-1 family are principal mediators of the acute posttraumatic infl ammatory response [36,37]. Increased IL-1 expression has been documented after mechanical joint injury and correlates with the severity of cartilage damage [38]. IL-1 is overexpressed by chondro- cytes, synoviocytes and infi ltrating infl ammatory cells. Furthermore, synovial fl uid levels of the IL-1 receptor antagonist (IL-1Ra) decrease after ACL injury [39]. IL-1 induces mediators of joint pain, and it promotes cartilage matrix degradation by inducing expression of extra- cellular matrix-degrading enzymes and inhibiting extra- cellular matrix synthesis and the anabolic activity of growth factors [40,41].  e levels of IL-6 and TNFα in human synovial fl uid also increase signifi cantly after acute joint injury [11,42]. IL-6 with its soluble receptor potentiates the catabolic eff ects of TNFα in the degradation and loss of proteo glycans from cartilage [43]. Furthermore, mechanical injury potentiates proteoglycan catabolism induced by this combination of TNFα and IL-6 with its soluble receptor [44].  is provides a potential Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 3 of 9 mechanism linking the immediate and acute events following trauma. Extracellular matrix-degrading enzymes Release of extracellular matrix-degrading enzymes has been established as an important mechanism in post- traumatic cartilage damage. Fragments of extracellular matrix such as collagen or fi bronectin fragments that are generated by these enzymes stimulate further production of pathogenetic mediators [45].  e specifi c enzymes, kinetics of release and cellular origin vary with the experimental model used. Studies with cartilage explants that are subjected to mechanical impact injury demon- strate that the remaining viable chondrocytes express increased levels of matrix metalloproteinase (MMP)-1, MMP-3, MMP-8, MMP-9, MMP-13 and ADAM-TS5 [46,47]. Analyses of synovial fl uid samples taken from patients after an ACL or meniscal tear revealed increased MMP-3 levels that remained elevated for many years [48]. Joint fl uid also showed an initial and persistent elevation of the neoepitope Col2CTx in the C-telopeptide of type II collagen, indicating digestion of mature, cross- linked collagen by a MMP. Fragments of cartilage oligo- meric protein and aggrecan were also elevated [49-51]. Taken together, these studies suggest that extracellular matrix degradation rates are signifi cantly altered within days of the injury and remain altered for years.  e acute joint tissue response to the original mechanical insult thus seems to initiate an unbalanced degradative process that can signifi cantly increase the risk of OA. Pharmacological treatment options  ere is a clear recognition of the risk to develop OA after joint trauma, and thus there is an obvious and urgent need to develop and implement strategies that prevent posttraumatic cartilage degradation. Here we focus on pharmacological interventions, but these need to be integrated with surgery and neuromuscular–bio- mechanical training. Research on pathogenetic mecha- nisms has identifi ed major pathways and therapeutic targets. Pharmacological interventions need to inhibit the posttraumatic infl ammatory responses, prevent cell death, prevent degradation and stimulate production of new cartilage extracellular matrix.  e optimal therapy should address several or all of the pathogenesis path- ways. Whether separate approaches are required to fi rst interfere with early catabolic and infl ammatory events and subsequently to promote anabolic responses, so that therapeutic approaches eff ectively stimulate proper cartilage repair at the appropriate time after trauma, needs to be deter mined. Goals of therapy are to provide immediate and long-term benefi ts, and it is possible that interventions during the fi rst few months after injury can accomplish both. An important unanswered question is when and which therapies that have been developed as disease-modifying OA drugs [52] are indicated for patients with post traumatic OA. A promising route of drug administration during the early phase after joint injury is intraarticular injection.  is has the advantages of reaching high drug concentrations at the lesion site with low systemic drug exposure, and thus reduced risk for systemic adverse events. Animal models used to test potential therapies include joint damage and OA-like pathology induced by creating joint instability through performing ligament transection and/or meniscectomy.  ese models are associated with chronic or repetitive impact loading, and lead to a rapid develop ment of full thickness cartilage lesions within 3 to 8weeks. Such models are the standard tools to evaluate disease-modifying OA drugs, and have been used to identify a large number of therapies that improve experi- men tal lesions. A limitation of repetitive injury as a model of posttraumatic OA is that it disrupts endogenous repair responses. Single closed-impact injuries probably repre- sent better models of human joint trauma.  e closed- impact models are performed in larger animals [53,54]. Caspase inhibitors Evidence from in vitro and animal model studies suggests there is a time window after injury when cartilage cells can be rescued or protected from undergoing cell death, resulting in the maintenance of viable and functional cells and reducing cartilage structural damage [25].  is off ers the opportunity of preventing chronic joint destruction, pain and disability by intraarticular administration of a pharmaceutical during the immediate time interval after joint injury.  e key role that caspases play in initiating and executing apoptosis make them prime targets for apoptosis modulation. Antiapoptotic agents have been successfully explored in models of diseases aff ecting the central nervous system, liver and kidneys [55-57]. Speci- fi c to chondrocytes, a series of in vitro studies demon- strated that caspase inhibitors are eff ective in protecting against chondrocyte apoptosis, maintaining viable and functional cells [58]. Reduction of cartilage degeneration following intraarticular injection of caspase inhibitor has also been reported for a rabbit model of OA [59]. Caspases recognize substrates with a strict requirement for aspartic acid. Caspase inhibitors have been developed on dipeptide, tripeptide and tetrapeptide scaff olds that represent recognition sites in caspase substrates and a fl uoromethylketone warhead [60].  e inhibitors diff er in their specifi city for individual caspases, in their ability to penetrate into the intracellular space and in whether they are reversible or irreversible inhibitors. In addition to the role of caspases in the regulation and execution of cell death, caspase 1 (also termed IL-converting enzyme) is responsible for converting the precursors of IL-1 family Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 4 of 9 cytokines IL-1β and IL-18 to their active form. Pharma- ceu ticals that inhibit IL-converting enzyme/caspase-1 thus have potential to neutralize the pathogenic eff ects of IL-1 family cytokines [61]. Chemical caspase inhibitors are available that are specifi c for individual caspases or neutralize the activity of all caspases. Such pan-caspase inhibitors would be ideally suited to be eff ective as interventions for the acute posttraumatic infl ammation and to limit cell and cartilage damage.  e pan-caspase inhibitor z-VAD.fmk is a prototype compound that has been used extensively in vitro and in animal models for proof-of-concept studies. Inhibitors of specifi c caspases as well as pan-caspase inhibitors were tested in various models, and pan-caspase inhibitors appeared to be most potent in reducing chondrocyte apoptosis and GAG release [62,63]. A similar compound – a dipeptide-based, irreversible, cell- permeable and broad-spectrum caspase inhibitor [64] – was evaluated in the treatment of liver disease and results from phase II clinical trials were published [65].  e drug showed no adverse eff ects and improved markers of liver damage in patients with chronic hepatitis C virus infection [65]. Proof-of-concept has thus been established for caspase inhibition as an eff ective therapy for diseases where tissue damage is related to cell death. Candidate drugs with established clinical safety are available for testing in posttraumatic arthritis. Cytokine inhibitors, anti-in ammatory cytokines IL-1 inhibition, mainly through the use of IL-1Ra, is therapeutically eff ective in animal models of OA [66,67], and preliminary observations from a clinical trial in patients with OA suggest symptom-modifying activity [67]. In antigen-induced arthritis in rabbits, IL-1Ra also had a profound antifi brotic eff ect [68]. In this model, the synovial fi brosis was not only halted by administration of IL-1Ra but it was reversed [16]. Diacerhein, which inter- feres with the infl ammatory and catabolic eff ects of IL-1, provided nearly complete protection in impact models [69]. TNFα inhibition by subcutaneous injection of a soluble TNF receptor fusion protein showed disease- modifying activity in the anterior cruciate ligament transection model of posttraumatic arthritis in rats [70].  e anti-inflammatory cytokine IL-10 has a spectrum of chondroprotective activities in chondrocytes. It stimu- lates collagen type II and proteoglycan expression, inhibits MMP, proinflammatory cytokine or nitric oxide expression and protects against chondrocyte apoptosis (reviewed in [71]). IL-10 has also been therapeutically eff ective in an experimental animal model of early OA [72].  e chondroprotective potential of IL-4 has been demonstrated recently [73].  ese observations suggest therapeutic potential of anti-inflammatory cytokines in posttraumatic cartilage damage [74]. Growth factors Bone morphogenetic proteins (BMPs) are potent stimuli of mesenchymal cell diff erentiation and extracellular matrix formation. BMP-7, also termed osteogenic protein-1, has been studied extensively in vitro as well as in animal models, and results suggest that BMP-7 may be a candi- date as a disease-modifying OA drug and also for post- traumatic arthritis. Unlike transforming growth factor beta and other BMPs, BMP-7 upregulates chondro cyte metabolism and protein synthesis without creating uncontrolled cell proliferation and formation of osteo- phytes. BMP-7 prevents chondrocyte catabolism induced by IL-1, fi bronectin fragments or hyaluronan hexasaccha- rides. BMP-7 has synergistic anabolic eff ects with other growth factors such as insulin-like growth factor-1, which in addition to its anabolic eff ect acts as a cell survival factor (reviewed in [75]). Insulin-like growth factor-1 has chondroprotective activity in various animal models [76]. In acute chondral defect models in the dog [77] and goat [78], BMP-7 regenerated articular cartilage, increased repair tissue formation and improved inte- grative repair between new cartilage and the surrounding articular surface. Fibroblast growth factors (FGFs) are important regulators of cartilage development and homeostasis [79]. FGF-2 can stimulate cartilage repair responses [80], but its potent mitogenic eff ects may lead to chondrocyte cluster formation and poor extracellular matrix due to a relatively low level of type II collagen [79]. In a rabbit anterior cruciate ligament transection model, however, sustained release formulations of FGF-2 reduced OA severity [81]. FGF-18 has anabolic eff ects on chondro- cytes and chondroprogenitor cells, and stimulates cell proliferation and type II collagen production [82]. In a rat meniscal tear model of OA, intraarticular FGF-18 injections induced a remarkable formation of new carti- lage and reduced the severity of the experimental lesions [83]. FGF-18 and BMP-7 are currently in clinical evaluation in patients with established OA. Inhibitors of extracellular matrix-degrading enzymes A large number of matrix-degrading enzymes – including MMPs, aggrecanases or cathepsins – are involved in cartilage matrix destruction in OA, and inhibitors have been tested extensively in OA animal models. Several MMP inhibitors have been evaluated in clinical trials in patients with established OA, and failed either because of adverse events or lack of effi cacy.  e most common adverse event was termed musculoskeletal syndrome – fi brotic lesions due to interference of the inhibitors with normal collagen turnover [52].  is may not represent a major risk if such drugs are administered intraarticularly or for short periods of time, for example, to limit the Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 5 of 9 irreversible collagen degradation in the fi rst few months after injury. Enzyme inhibitors have not been tested in single-impact animal models, but in cartilage explants a MMP inhibitor reduced GAG loss between 1 and 7 days post injury [14]. Antioxidants Traumatic cartilage and joint injury is associated with increased production of reactive oxidant species and reduced antioxidant defenses, and this imbalance contri- butes to cell death and degradation of extracellular matrix [84]. Chondrocyte death induced by mechanical injury was reduced by antioxidants such as the superoxide dismutase mimic Mn(III)porphyrin [85], vitamin E and N- acetylcysteine [86]. Moreover, N-acetylcysteine treatment signifi cantly improved proteoglycan content at the impact sites [87]. Brief exposure to free radical scavengers could thus signifi cantly improve chondrocyte viability and protect against extracellular matrix damage following joint injury. Aminosugars Glucosamine is being used by a large number of OA patients but discussion of its effi cacy and mechanism of action after oral administration continues. High concen- trations of glucosamine and related aminosugars, how- ever, have anabolic and anti-infl ammatory eff ects on chondrocytes and other joint tissue cells [88]. Since such high concentrations in joints can presumably not be attained after oral administration, intraarticular injec- tions may represent a feasible and eff ective approach. Among the various aminosugars that have been tested, N-acetylglucosamine has a superior spec trum of activities in vitro [89]. Intraarticular injection of N- acetylglucosamine was also eff ective in an animal model of OA [90]. Joint lubricants Hyaluronan and lubricin are important lubricants of the cartilage surfaces. Lubricin concentrations in synovial fl uid are reduced in patients with traumatic arthritis, due to enzymatic degradation and suppression of its synthesis by infl ammatory cytokines [11,91]. In rats with meniscal tear-induced OA, intraarticular injections of recombinant lubricin resulted in disease-modifying, chondroprotective eff ects [92]. Similar to lubricin, hyaluronan is degraded in infl amed joints and there are numerous reports of chondroprotective activities in experimental models of OA [93]. Interestingly, both lubricin and hyaluronan have activities beyond lubrication that may be benefi cial in the setting of posttraumatic arthritis. Conclusion OA is the most common form of joint disease aff ecting a patient population that is heterogeneous with regard to risk factors and stage of the disease. Disease-modifying OA therapies are presently not available. Approximately 50% of patients with traumatic joint injury develop OA and represent a subset of OA that is readily identifi ed and accounts for approximately 12% of all OA cases. Unmet needs exist to address the acute posttraumatic infl am ma- tion and to prevent or delay the development of OA. Research on experimental models of posttraumatic OA and clinical research has led to the elucidation of patho- genesis pathways.  e ideal therapy must be multi-varied and include positive eff ects on chondrocyte metabolism and stimulation of intrinsic repair while inhibiting catabolic pathways that lead to chondrocyte death and matrix loss. A series of molecular targets and drug candidates have been identifi ed, and many of these drug candidates were eff ective in animal models of joint injury and OA (Table2).  e current challenge and opportunity is in the translation of this information into eff ective therapies (Table 3).  e major challenge is the long time interval, ranging from 5 to 15 years, between joint trauma and OA-like joint pathology in humans as detected on radiographs or magnetic resonance imaging. Since it is not feasible to conduct clinical trials of such duration, any therapy to be introduced into clinical use will thus Table 2. Potential targets and drugs for pharmacological intervention in posttraumatic arthritis Target Drugs References Caspases Small molecule inhibitors [59,60,63,64] Proin ammatory cytokines (TNFα, IL-1, IL-6) Neutralizing antibodies, IL-converting enzyme, TACE inhibitors [66-70] Cartilage repair Growth factors (BMP-7, FGF-2, FGF-18, IGF-1) [75-83] Matrix-degrading enzymes (matrix metalloproteinases, Small-molecule inhibitors, TIMP [14] aggrecanases, cysteine-dependent cathepsins, neutrophil-derived enzymes) Oxygen radicals SOD, SOD mimetics [84-87] Lubricant de ciency Lubricin, hyaluronan [92,93] Anti-in ammatory cytokines [71-74] SOD, superoxide dismutase; TACE, TNFα converting enzyme; TIMP, tissue inhibitors of metalloproteinases. Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 6 of 9 depend on demonstrating effi cacy on the basis of surrogate markers, such as biochemical markers that predict or correlate with the progression of cartilage and joint damage. Although candidate markers have been identifi ed, they need further validation. A need also exists for therapies that address the acute joint infl ammation and improve subjective symptoms such as pain, stiff ness and joint dysfunction during the fi rst 2 to 3 months after injury. Clinical trial design for such studies is facilitated by the availability of established endpoints for joint pain, function and infl ammation. Based on the notion that the original lesion expands rapidly during this time, there is an opportunity to simultaneously address symptoms and limit lesion expansion. We propose, as a near-term approach, interventions that should be tested as soon as possible after joint trauma with primary objectives to reduce pain and infl ammation and with secondary objectives to improve biomarkers of joint destruction. Such therapies can be administered as injections into the aff ected joints, and have advantages of reduced risk for systemic adverse events and reaching high drug levels at the target tissues. Formulation technologies are available to extend intra- articular retention and thus limit the number of injections [94]. Several drug candidates have already been tested extensively in preclinical models, and some candidates have already been in human clinical trials for established OA or other indications. Abbreviations ACL, anterior cruciate ligament; BMP, bone morphogenetic protein; FGF,  broblast growth factor; GAG, glycosaminoglycan; IL, interleukin; IL-1Ra, IL-1 receptor antagonist; MMP, matrix metalloproteinase; OA, osteoarthritis; TNF, tumor necrosis factor. Acknowledgements The present work was supported by NIH grants AR058954 and AG007996. Competing interest The author declares that he has no competing interests. Published: 28 June 2010 References 1. Buckwalter JA, Thomas TD: Joint injury, repair, and remodeling: roles in post-traumatic osteoarthritis. Clin Orthop Relat Res 2004, 423:7-16. 2. Brown TD, Johnston RC, Saltzman CL, Marsh JL, Buckwalter JA: Posttraumatic osteoarthritis: a  rst estimate of incidence, prevalence, and burden of disease. J Orthop Trauma 2006, 20:739-744. 3. Roos EM: Joint injury causes knee osteoarthritis in young adults. Curr Opin Rheumatol 2005, 17:195-200. 4. 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Posttraumatic osteoarthritis: needs, opportunities and challenges Therapies needed for posttraumatic in ammation and osteoarthritis prevention Readily identi ed population at high risk for osteoarthritis Pathways and targets identi ed Several clinical-stage therapeutic candidates Therapeutic window immediately following injury Long latency period until overt structural change Clinical trials for osteoarthritis prevention depend on surrogate markers This article is part of a review series on New developments in osteoarthritis, edited by Martin Lotz and Stefan Lohmander. Other articles in the series can be found online at http://arthritis-research. com/series/osteoarthritis Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 7 of 9 patellar tendon transplantation. Modi cation by surgical technique and surgery timing. Unfallchirurgie 1995, 21:240-246. 24. 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Flannery CR, Zollner R, Corcoran C, Jones AR, Root A, Rivera-Bermudez MA, Blanchet T, Gleghorn JP, Bonassar LJ, Bendele AM, Morris EA, Glasson SS: Prevention of cartilage degeneration in a rat model of osteoarthritis by intraarticular treatment with recombinant lubricin. Arthritis Rheum 2009, 60:840-847. 93. Axe MJ, Shields CL, Jr: Potential applications of hyaluronans in orthopaedics: degenerative joint disease, surgical recovery, trauma and sports injuries. Sports Med 2005, 35:853-864. 94. Larsen C, Ostergaard J, Larsen SW, Jensen H, Jacobsen S, Lindegaard C, Andersen PH: Intra-articular depot formulation principles: role in the management of postoperative pain and arthritic disorders. J Pharm Sci 2008, 97:4622-4654. doi:10.1186/ar3046 Cite this article as: Lotz MK: Posttraumatic osteoarthritis: pathogenesis and pharmacological treatment options. Arthritis Research & Therapy 2010, 12:211. Lotz Arthritis Research & Therapy 2010, 12:211 http://arthritis-research.com/content/12/3/211 Page 9 of 9 . methemoglobin, and hemosiderin appear to mediate the blood-induced damage. Hemarthrosis also leads to synovial hypertrophy and siderosis, due to phagocytosis of erythrocytes and hemoglobin by synovial. of steroid injections, physical therapy and, ultimately, surgery for debridement. Arthrolysis surgery may be required more than once in some patients. A key strategy for the prevention of arthrofi. joint injury and correlates with the severity of cartilage damage [38]. IL-1 is overexpressed by chondro- cytes, synoviocytes and infi ltrating infl ammatory cells. Furthermore, synovial fl uid